Magnesium hydroxide has been utilized in the desulfurization of flue gas and neutralization of effluent water. This hydrated magnesia is produced using sea water magnesium chloride as the starting material. However, the magnesium content of sea water is so low that the production process involves a complicated series of steps. Moreover, since a magnesium hydroxide Slurry is highly viscous even at a low concentration of about 30 weight %, the costs of production and transportation are high.
On the other hand, a process involving the hydration of light burned or calcined magnesia obtainable by burning naturally-occurring magnesite is known. Japanese Patent Publication No. 60774/1991, for instance, discloses a process for producing active magnesium hydroxide which comprises pulverizing calcined natural magnesite to provide powdery light burned magnesia and hydrating this magnesia with water at pH not less than 11 and an elevated temperature of not less than 85.degree. C. In this process, the solubility in water and the rate of hydration of magnesia are so high that an active magnesium hydroxide exhibiting high rates of reaction with acidic substances can be obtained.
However, this process requires not only a protracted time for the pulverization of light magnesia but also requires a large-scale plant for mass production. Furthermore, in order to insure a sufficiently high hydration rate, it is necessary to finely divide light magnesia beforehand and classify the powder to the particle diameter of the order of about 1 to 30 .mu.m. Moreover, fine particles prepared by such pulverization and classification operations contaminate the working environment, thus presenting safety and health problems. In addition, the particle size distribution of the resultant magnesium hydroxide is so broad, e.g. about 1 to 40 .mu.m, that a quality-wise Uniform magnesium hydroxide is hardly obtained.
When exposed to moisture or water, fine particles of light magnesia tend to aggregate into blocks. Once blocks are formed, it is difficult to hydrate them uniformly in a short period of time.
Furthermore, since the production process involves pulverization, classification and hydration steps, the productivity can hardly be increased.